It has long been known to use punch-and-die technology to form a part into a desired shape. More specifically, the die provides the desired shape, and the punch is used to force the material of the part to conform to the shape of the die and thereby transfer the shape to the material.
More recently, a process of hydroforming has been developed and used in which fluid pressure performs the function of the conventional punch in forcing the material to conform to the shape of the die. The hydroforming process advantageously allows for producing parts or other components as single structures that would otherwise have been made from multiple stampings joined together. Producing a part as a single structure rather than multiple joined structures allows for substantial weight savings by eliminating flanges necessary for welding, fastening, or otherwise joining the multiple structures, and by using thinner steel or other material while maintaining overall stiffness through the elimination of discontinuous spot-welded joints. Producing a part as a single structure also allows for substantial labor savings by eliminating the labor required to weld, fasten, or otherwise join the multiple structures.
Hydroformed tubular structures, such as are used in the automotive industry, are formed from a single material such as mild steel or an aluminum alloy. Because both mild steel and aluminum alloys each have particular inherent characteristics with respect to such factors as, for example, weight, density, strength, flexural stiffness, and cost, it can be difficult to satisfy often conflicting requirements in body structure design when only a single material is used.
Thus, a method of producing a tubular structure from two or more materials is needed.